What does the FTG designation for WROT copper fittings mean?

The FTG designation for WROT copper fittings indicates that the end is the same diameter as the copper tube (CTS) it is designed to be inserted into, another copper solder socket or brass solder valve.

This is similar to the term SPIGOT (SPG abbreviation) used for Sch. 40 & 80 PVC fittings.